The tissue examined consisted of both renal cortex and medulla with adequate numbers of glomeruli for review. The vast majority of glomeruli were non-sclerotic and showed diffuse involvement by a non-uniform, focally nodular, hypocellular mesangial expansion, and focal capillary wall thickening by a waxy material, suggestive of amyloid protein (Image 09 - low power H&E). Approximately 10% of glomeruli were globally sclerotic, and multifocal diffuse lymphoplasmacytic infiltrates were noted (Image 10 - H&E). The involved glomeruli on high power showed dense amorphous, primarily mesangial deposits that focally simulate thrombi in some peripheral capillary loops (Image 11). PAS (not shown) and Congo red stains highlighted the glomerular mesangial deposits (Images 12 and 13). Similar congophilic material was also present as diffuse deposits within the interstitium (Image 14), and seen within the walls of intrarenal segmental arteries and arterioles demonstrating marked mural thickening, and apple-green birefringence under polarized light (Image 15). An panel of immunostains for amyloid protein (not shown) and immunofluorescence results for kappa and lambda light chains, further characterized the glomerular and vascular deposits to be positive for amyloid P (the common precursor protein to all forms of amyloid), and lambda light chain. Immunofluorescence stains for IgM showed 2-3+ segmental glomerular staining (Image 16), and 3-4+ similar staining for lambda light chain (Image 17). Segmental arteries also showed strong 3+ staining by fluorescence for lambda light chain (Image 18). The renal tubules demonstrated moderate to focally marked atrophy, and were associated with a moderate amount of interstitial fibrosis. Focal tubular hyaline casts were present, but no myeloma casts were seen. No necrotizing glomerular lesions, crescents, vasculitis, or thromboemboli were noted.

Multiple dense interstitial lymphoid aggregates composed of small mature lymphocytes and lymphoplasmacytic cells were also present (Image 19). As seen in the bone marrow, the infiltrating cells were predominantly CD20 positive (Image 20) and lambda light chain restricted (Image 21). An immunostain for IgM highlighted the glomerular deposits and the cellular infiltrates, similar to the lambda light-chain immunostain (Image 22). Scattered admixed reactive CD3 and CD43 positive T-cells were also noted in smaller numbers.